As an example of information device for optically detecting information according to the prior art, an optical disk device will be described. The optical disk device irradiates an optical disk, which is its recording medium, with forward light emitted from a laser beam source and detects variations in the intensity of the reflected backward light. Then, information recorded on the optical disk can be detected on the basis of these variations in the backward light. This kind of information device requires keeping the spot shape of the light beams appropriate to ensure highly reliable reproduction or recording. For this purpose, it is essential to suppress wave front aberrations of the light beams arising on the optical path.
In an optical disk device, wave front aberrations may arise from any of a wide variety of causes including a tilt of the optical axis of the light beams relative to the recording layer of the disk, a variation in the thickness of the disk substrate and the smear of the disk surface with a fingerprint or the like, and the type of wave front aberration may vary with the cause. For instance, wave front aberrations due to tilts are predominantly coma aberrations and astigmatisms, while wave front aberration due to a variation in the thickness of the disk substrate are predominantly spherical aberrations.
According to the prior art, techniques of detecting a specific type of aberration from the output signals of an optical pickup and correcting it have been proposed, and they are disclosed, for instance, in the Japanese Patent Laid-Open No. 2000-155979. The information device disclosed in this patent application will be described below with reference to FIGS. 26(a) and 26(b).
In this information device, as shown in FIG. 26(a), light beams emitted from a light source 101, such as a semiconductor laser, are transmitted by a wave front converting element 304, after passing a half mirror 302 and being converted into substantially parallel light by a collimating lens 303. The transmitted light is subsequently brought to incidence by an objective lens 305 onto the write/read information layer through the substrate of an optical disk 306.
The light beams reflected by the write/read information layer of the optical disk 306 are again transmitted by the substrate, and transmitted successively by the objective lens 305, the wave front converting element 304 and the collimating lens 303. After they are reflected by the half mirror 302, they are diffracted by a hologram 309 and brought to incidence on an optical detector 307 for signal detection use.
The optical detector 307 is configured of optical detecting elements, such as pin diodes, for detecting information signals, control signals including focusing signals and tracking signals, and aberrations of light beams. These detecting elements may either be individually configured for different modes of signal detection or perform a plurality of functions by integrating them. The detected aberrations are processed by a signal processing circuit 308 and drive the wave front converting element 304.
The wave front converting element 304 is an element comprising two glass substrates between which liquid crystals are sealed in. In the wave front converting element 304, the part which light beams pass are divided into a plurality of areas, and a voltage is applied to each area independently of others to vary the refractive index of the corresponding part and thereby to change the phase of the wave front.
A configuration for detecting coma aberrations in particular, out of different wave front aberrations, is shown in FIG. 26(b) to exemplify the optical detector 307 and the hologram 309. Out of light beams 312 on the backward path, on which the beams reflected by the optical disk are condensed, only those passing the path center part 313 of the area of Y>0 are separated from the light beams passing other areas than the area 313, and condensed into bisected optical detectors 317a and 317b to cause a light spot 314 to be formed. Here, the configuration is such that the light spot 314 be formed to focus on the line dividing the optical detectors 317a and 317b from each other when there is no aberration. The area 313 is so set that, when any coma aberration has arisen in the direction of the Y axis, light beams ahead of or behind the light beams passing other areas than this in phase can be extracted.
If the light beams passing the area 313 are behind in phase, those light beams will be focused behind the detection surfaces of the optical detectors, and the output of the optical detector 317a will become greater than that of the optical detector 317b. Conversely, if the light beams passing the area 313 are ahead in phase, those light beams will be focused ahead the detection surfaces of the optical detectors, and the output of the optical detector 317a will become smaller than that of the optical detector 317b. By detecting a signal of the output difference between the bisected optical detectors 317a and 317b, the quantity and sign of the coma aberration are determined.
As another example of the optical detector 307 and the hologram 309, a configuration for detecting spherical aberrations in particular is shown in FIG. 26(c). An optical axis 310 is supposed to pass the origin of the X-Y coordinate system. In light beams 322 on the backward path, on which the beams reflected by the optical disk are condensed, only those passing the area 323 of Y>0, out of the areas between two concentric circles of differing diameters around the optical axis 310 are separated from the light beams passing other areas than the area 323, and condensed into bisected optical detectors 317a and 317b to cause a light spot 324 to be formed. Here, the configuration is such that the light spot 324 be formed to focus on the line dividing the optical detectors 317a and 317b from each other when there is no aberration. The area 323 is so set that, when any spherical aberration has arisen, light beams ahead of or behind the light beams passing other areas than this in phase can be extracted.
If the light beams passing the area 323 are behind in phase, those light beams will be focused behind the detection surfaces of the optical detectors, and the output of the optical detector 317a will become greater than that of the optical detector 317b. Conversely, if the light beams passing the area 323 are ahead in phase, those light beams will be focused ahead the detection surfaces of the optical detectors, and the output of the optical detector 317a will become smaller than that of the optical detector 317b. By detecting a signal of the output difference between the bisected optical detectors 317a and 317b, the quantity and sign of the spherical aberration are determined. Other aberrations, for instance astigmatisms, can be detected if the arrangement and shapes of the optical detector 307 and the hologram 309 are optimally designed.
Besides the liquid crystal system, there is also available for the wave front converting element 304 a deformable mirror system by which the optical path length is controlled by deforming a flexible deformable mirror, and this technique is disclosed in the Japanese Patent Laid-Open No. 11-14918. The configuration here is such that a metallic thin film is vapor-deposited on the surface of a flexible deformable plate to make it a mirror surface, an electrode is provided on the rear side of the deformable plate in the opposite position with a prescribed gap in-between, and the deformable plate is attracted with an electrostatic force by applying a voltage to this electrode.
The information devices described above involve the following problems.
First, as the detection and correction of wave front aberrations are limited to a specific type or another, it is not possible all the time to address a wave front aberration of a desired type. For instance if the hologram 309 and the optical detector 307 are so designed as to permit detection of coma aberrations in the direction of the Y axis as shown in FIG. 26(b), no other wave front aberrations (such as coma aberrations in the direction of the X axis, spherical aberrations and astigmatisms) can be detected accurately. The same is true of the case shown in FIG. 26(c), wherein no others than spherical aberrations can be detected accurately. However, usual wave front aberrations arise from combinations of causes including a tilt, a variation in the thickness of the disk substrate, double refraction and smear with a fingerprint or the like, but the aberrations that arise are not always confined to any specific type.
Accurate detection of such a broad variety of wave front aberration types is difficult with a configuration for aberration detection based on a specific pattern of wave front aberration. Similarly, in correcting wave front aberrations, the electrode pattern for drying liquid crystals for correcting coma aberrations and that for correcting spherical aberrations differ from each other, and accurate detection of a broad variety of wave front aberration types is difficult with a configuration for aberration detection based on a specific pattern of wave front aberration.
Second, it is difficult to achieve at the same time a broad enough range of wave front correction and adequate responsiveness and accuracy. In the case of a wave front converting element using liquid crystals, it is possible to widen the range of wave front correction by thickening the liquid crystal layer, but this would invite a deterioration in correction accuracy due to a drop in transmission efficiency and response speed, and increased discontinuity of the optical path length between electrode patterns. On the other hand, in a wave front converting element using a deformable mirror, as the only electrostatic force working on the deformable plate is an attracting force, there is a problem that in the configuration according to the prior art, active drive is only possible in the direction of bringing the deformable plate towards the electrode. Therefore, the only driving force in the reverse direction is only a passive one deriving from the righting force of the deformable plate which has been once deformed, and this lack of symmetry of the driving forces results in poor control accuracy and responsiveness. Moreover, bidirectional driving by utilizing such a passive force inevitably requires the use of a position deformed in advance to some extent as the reference face, it is made difficult to reproduce the shape of the reference face stably by fluctuations in drive sensitivity from one unit to another, and this invites a deterioration in the accuracy of correction.
The main object of the present invention is to provide a deformable mirror adaptable to a broad variety of aberration types and capable of correcting wave front aberrations, which is accurate, broad in correctable range and highly responsive and an information device equipped with such a mirror.
Another object of the invention is to provide an optical compensation device and a wave front detection device which can be readily reduced in size and cost and excels in relative positional accuracy.